1. Field of Invention
This invention relates generally to the actuator which includes the read/write head in an optical storage system and, more particularly, to the mechanism for supporting the actuator relative to the rest of the optical storage system. The actuator of the optical storage system must be capable of smooth movement, i.e., without sticking, and precise positioning.
2. Description of the Related Art
The requirements for the coupling of the actuator to the remainder of an optical storage and retrieval system suggest characteristics which have been found for magnetic field bushings. Magnetic bushings are commercially available and facilitate rotational movement. Typically, the bushings support rotating shafts. In FIG. 1A and FIG. 1B, commercially available magnetic field bushing designs are illustrated. In FIG. 1A, magnetic bushing 10 has a stator 11 which is surrounded by the rotor 12. The stator 11 has structures upon which conducting wire has been wound. The windings are arranged into coil 1, coil 2, coil 3, and coil 4. The rotor 12 and regions of the stator 11 associated with each coil are fabricated from a high permeability magnetic material. Associated with each coil is a plurality of air gaps 13 configured so that coil 1, coil 2, coil 3, and coil 4 are portions of electromagnet 14A, electromagnet 14B, electromagnet 14C, and electromagnet 14D, respectively, each electromagnet exerting a force on the rotor 12 determined by the number of windings in the associated coils and the current through the coils. In FIG. 1B, magnetic bushing 15 is configured so that the position of the stator 17 and the rotor 16 are reversed. The stator 17 has structure upon which electrical conductors are wound to form coil 1, coil 2, coil 3, and coil 4. The coils 1, 2, 3, and 4 along with the materials from which the stator regions and the rotor are fabricated and the air gaps 18 associated with each coil form electromagnets 19A, 19B, 19C, and 19D, respectively. A current through the windings will create a force between the stator 17 and rotor 16.
Referring to FIG. 2, an enlarged perspective view of an electromagnet 19A of magnetic field bushing 15 is shown. When the two windings have current flowing therethrough, the magnetic flux generally follows a path illustrated by broken line 21. As is well known, the magnetomotive force is given by: EQU F=.phi.H.multidot.dl amp.multidot.turns,
where H is the magnetic field intensity.
The magnetomotive force is generated by the current in the two windings of the coil. Therefore, EQU .phi.H.multidot.dl=2N.multidot.i
where
N=the number of turns in each winding; and
i=the current flowing through both windings.
Because the permeability of the material used to fabricate the stator and the rotor is much higher when compared with air, for two air gaps 18 of length 1, EQU 2N.multidot.i=2H.multidot.l
or EQU H=N.multidot.i/1 amp/m.
The magnetic flux density is then EQU B=.mu..sub.o H=.mu..sub.o N.multidot.i/1 weber/m.sup.2.
where .mu..sub.o is the permeability of free space=4.pi..multidot.10.sup.-7 webers/amp.multidot.m. Furthermore, the magnetic pressure is B.sup.2 /2.mu..sub.o newtons/m.sup.2, the magnetic pressure resulting in a force of attraction between the faces of the air gap equal to EQU F=A B.sup.2 /2.mu..sub.o newtons,
where A is the area of the air gap face. For a rectangular face, EQU F=b.multidot.h.multidot.B.sup.2 /2.mu..sub.o newtons
where
b=width shown in FIG. 2; and
h=height shown in FIG. 2.
By way of example, if N=50 turns, 1=0.508.multidot.10.sup.-7 m (0.020"); b=0.635.multidot.10.sup.-3 m (0.250"); h=12.7.multidot.10.sup.-3 m (0.500"); and i=1 amp; then F=0.491 newtons.
Referring to FIG. 3, for the coil placement shown in FIG. 1A and FIG. 1B, the rotor can be suspended, if the forces acting on the rotor satisfy the following conditions: EQU F.sub.2 =F.sub.4 EQU F.sub.3 =F.sub.1 +mg
where mg is the force of gravity exerted on the rotor 17. As indicated in the discussion with respect to FIG. 2, the forces are a function of the current passing through each of the coils. As a practical matter, the stator typically includes a plurality of sensors (not shown in FIG. 1 or FIG. 2). These sensors provide a signal related to the air gap distance, i.e., the distance between the stator and the rotor. This sensor signal is used to control the current through the coils by means of a feedback loop and therefore controls the force between the stator and the rotor. The resulting separation of the stator and the rotor can be maintained in this manner.
As will be clear to those skilled in the art, the magnetic field bushing described above has two degrees of freedom, the first degree of freedom being an axial rotation and the second degree of freedom being motion in the axial direction. Because no contact is present between the stator and the rotor, the coefficient of static friction between the components is absent. Similarly, no force exists in the axial direction so that a relatively modest force is necessary to move the rotor relative to the stator and the response to a force is limited only by the mass of component mechanically coupled to the rotor. These characteristics of the magnetic field bushing suggest that application to the radial motion of an actuator in a read/write head or actuator of an optical storage system would be appropriate. The actuator of an optical storage unit typically moves optical components required to interact with an optical medium, the medium typically having a disk configuration. The actuator must be rapidly brought into, and maintained, in precise alignment with a groove on the disk.
A need has therefore been felt for apparatus and associated method in which the advantages of the magnetic field bushing can be applied to the actuator of an optical storage system.